What can noble gases really say about mantle. 2) Extent of mantle degassing

Transcription

1 What can noble gases really say about mantle convection and the deep Earth volatile cycles? 1) Constraints on mass flow 1) Constraints on mass flow 2) Extent of mantle degassing

2 Outline: -Noble gas geochemistry Carbon helium relationship (magmatic degassing) Mass flow constraints

3 He isotope geochemistry Two isotopes of helium: 3 He and 4 He 3 He is primordial and 4 He produced by radioactive decay of U and Th U Th 238t 235 t t e 1 7 e 1 6 e He He U He He He He He 3 o Helium behaves as an incompatible element during mantle melting (i.e. prefers melt over minerals) Helium more incompatible than U-Th; low 4 He/ 3 He ratios reflect less degassed mantle material Helium not recycled back into the mantle

4 After Barfod, 1999 He isotope ratios from selected MORBs, OIBs and continental hotspots

5 But not a lot of helium in the residue. Partition coefficient argument is a redherring. He is not subducted but U and Th are subducted d back into the mantle.

6 After Barfod, 1999 He isotope ratios from selected MORBs, OIBs and continental hotspots MORBs: sample well-mixed degassed mantle with high U+Th/ 3 He OIBs: sample heterogeneous, less degassed mantle with high U+Th/ 3 He

7 Ne isotopic geochemistry Less degassed More degassed Increasing air contamination Radiogenic ingrowth Figure from Graham Ne generated by nuclear reactions involving i -particles, l neutrons, O, and Mg 21 Ne / 22 Ne varies because of radiogenic ingrowth and varying degrees of degassing. Different ocean islands have distinct 21 Ne/ 22 Ne ratios; either reflects varying amounts of MORB mantle addition to the OIB source(s) or different parts of the mantle have been degassed and processed to different degrees.

8 Geochemistry of Ar Three stable isotopes of Ar, 36 Ar, 38 Ar, 40 Ar 36 Ar and 38 Ar are primordial 40 Ar produced by radioactive decay of 40 K Ar is expected to be more incompatible than K during mantle melting If so high 40 Ar/ 36 Ar reflects degassed mantle material MORBs: 40 Ar/ 36 Ar 28,000-40, OIBs: 40 Ar/ 36 Ar 6,000-10,000. Air: 40 Ar/ 36 Ar 295.5

9 C He relationships C_ flux =CO 2 / 3 He * 3 He_ flux ; u C_ flux =CO 2 / 3 He * [ 3 He] x magma flux * (1/melt fraction); (Marty&Tolstikhin,1998) u CO 3 He 1KHe/ KCO2 ( ) / KHe KAr 2 CO2 He / Ar obs initial He measured He/ Ar inital e.g., Marty and Tolstikhin, 1995

10 MORB: 3 He flux ~ 1000 ± 250 moles/yr C/ 3 He ratio = 2.2 ± 0.7 x 10 9 C flux = 2.2 ± 0.9 x mol/yr Marty and Tolstikhin, 1995 CO 2 content in MORB source = ~150 ppm. Saal et al 2002: C flux = 9.3± 2.8x mol/yr 72 ± 19 ppm in MORBs from Siqueiros transform fault

11 For plumes: C flux magma flux 3 3 He C/ He F plume plume magma flux km 3 /yr C/ 3 He = 3 x 10 9 in plumes; F= 0.5 Assume 3 He in plume source ~ 3 He in MORB source; Cflux=3x10 x 12 mol/yr Marty and Tolstikhin, 1995 Uncertainty = huge

12 For arcs: C flux magma flux 3 3 He C/ He F UM arc magma flux km 3 /yr C/ 3 He = 1.1 ± 0.33 x in plumes; F= Assume 3 He in plume source ~ 3 He in MORB source; C flux = 2.5 x mol/yr Marty and Tolstikhin, 1995 Uncertainty = huge

13 Compilation of 3 He and CO 2 fluxes (mol/yr) of arc-related and global subaerial volcanism (Hilton et al., 2002). Species Arc Flux Reference Global Subaerial Flux Reference 3 He 200± ± ± > CO x ±0.58 (x ) ±0.2 (x10 12 ) x ±0.4 (x10 12 ) x x x x ±0.5 (x10 12 ) x x x CO 2 fluxes based on a combination of direct flux estimates and C/ 3 He ratios

14 o is observed, f is the fraction M is MORB mantle, L is carbonate sediment, S is organic sediments C/3He of 1.5 x 10 9, 1 x 10 13, 1 x 10 13, for M, L, and S, respectively. 13 Cof o o -30 o /oo, 0 /oo, and /oo for M, L, and S, respectively. Approximately 10-15% of the carbon derived from the mantle wedge and the rest from decarbonation reactions in the down going g slab.

15 Magma degassing: Control of CO 2 on noble gases MORBs Cannot be explained by simply invoking different eruption depths for MORBs and OIBs. 3 He e/ 22 Ne But recall Henry s Law: C s =K h P X OIBs log 10 [ 3 He cc g -1 ] Change CO 2 content and concentration of dissolved noble gases in the melt will change!!

16 Effects of varying CO 2 content and disequilibrium for eruption at a constant water depth (Gonnermann and Mukhopadhyay, 2007) He/ 22 Ne 3 H log 10 [ 3 He cc g -1 ]

17 He/ 22 Ne 3 H log 10 [ 3 He cc g -1 ] MORBs 0.07 to 0.24 wt % CO 2 with a mean of 0.13 wt% (130 ppm in source) OIBs 0.11 to 0.63 wt % CO 2 with iha mean of f wt % (360 ppm in source) So higher CO 3 2 contents lead to greater He loss for OIBs.

18 Summary Compared to undegassed MORB magmas, undegassed OIB lavas could be ~3-10 times enriched in 3 He. consistent with derivation from a gas rich source

19 Estimated CO 2 contents in undegassed magmas measured noble gases in erupted lavas water depth If MORBs represent the upper mantle and OIBs the lower mantle, then CO 2 content of mantle is ~ 300 ppm and BSE is ~357ppm

20 Last CIDER report on volatiles in the Earth - Saal et al 2009 Progress Report Conclusions: Approximate concentrations Depleted Mantle H 2 O 50 ppm; CO 2 20 ppm; Cl 1 ppm; F 7 ppm Enriched Mantle H 2 O 500 ppm; CO ppm; Cl 10 ppm; F 18 ppm Total Mantle H 2 O 366 ppm; CO ppm; Cl 7 ppm; F 15 ppm BSE Carbon content not likely to be 1000 ppm or higher (e.g., Javoy and Pineau, 1991).

21 Insights into the solid earth carbon cycle Predictions are based on our degassing model C_ flux =CO 2 / 3 He * 3 He_ flux ; One has to be very careful in using C/ 3 He ratios to infer C flux.

22 How do you preserve a reservoir with high concentrations of noble gases? dc C(1 f ) q dt (1 f ) qt C( t) C e C o o e qt C is concentration f is the fraction being recycled back q is no. of reservoir volumes processed per 4.5 Gy t is time (units of 4.5 Gy) For q =1 and for t = 1, ~ 40% of primordial volatile still left even after a mass equivalent to the mass of the reservoir is processed through melting.

23 A little more complicated approach: What happens if mixing is not instantaneous?

24 What happens if mixing is not instantaneous? 3 He/ 3 Heinitial in mantle reservoir r Here is the characteristic mixing time of the slab Bottom line: Substantial primordial gases can be preserved in a mantle reservoir that has been processed by partial melting. Even after a mass equivalent to 3-4 reservoir masses has been melted, a few percent of the original volatile inventory still left.

25 The 40 Ar concentration paradox (Allegre et al.,1996) Assuming ppm of K, 40 Ar produced over Earth history = x g 40 Ar in the atmosphere = 66 x g(~40-50%) 40 Ar in the crust = 9-12 x g x of 40 Ar has to be in the mantle So approximately 40-60% of the 40 Ar in the mantle. MORB 40 Ar concentration too low to account for this A simple reason for needing a large reservoir with Ar concentrations that is a factor ~10 higher than the MORB source.

26 A coupled upper-lower mantle n box model Gonnermann and Mukhopadhay, constraints (5 initial conditions 7 present day values) 12 constraints (5 initial conditions, 7 present day values) 9 free parameters (key ones mixing time, fraction of slabs subducted into lower mantle)

27 Solution space for 3 He? UM: 3-7 reservoir masses processed and Gy LM: reservoir masses processed and Gy

28 Evolution of Helium ratios and Nd isotopes over time ,000 21,000 85,000 4 He/ 3 He Solution can be found such that present day observational constraints from 3 He, 4 He/ 3 He ratios, 40 Ar concentration, Nd isotopes can be simultaneously satisfied. DMM FOZO Gonnermannn and Mukhopadhyay, 2009

29 Solutions exist such that 40-60% of Ar produced over earth history still in mantle majority of this in LM UM concentration from ocean He flux UM: 3-7 reservoir masses processed and Gy LM: reservoir masses processed and 0 2 Gy Somewhere between timesmassof mass mantle has passed through the melting column But limited direct mixing between upper and lower mantle

30 Solutions exist such that 40-60% of ar produced over earth history still in mantle majority of this in LM Mass flux into LM: One lower mantle mass over 4.5 Ga. UM concentration from ocean flux At present up to 30% of slabs subduct into LM; over Earth history ~50% of slabs have subducted into LM.

31 Over Earth history half the slabs go into LM. Present day a third of the slabs going onto LM. How does it match with present day heat flow constraints? Tank experiments Griffiths & Turner, 1988 Fra actional of flow tha at is slab Crowley and O'Connell (2010) Fraction of slab into lower mantle T T( o m K) (K) Lower mantle turn over time (Ga) 1-3 turnovers of the lower mantle mass need not result in a hot lower mantle.

32 A different view from geodynamic models. Geochemical predictions from geodynamic models (e.g., Brandenburg et al., 2008) ~20 lower mantle masses have fluxed through h the upper mantle Data Predictions from geodynamic model Fit okay but not great Happy medium somewhere between 1 and 20 lower mantle masses fluxing through the upper mantle?. Black =ambient mantle Red= CMB pools

33 Isotope arrays and mixing in the mantle 5 R A 8R A 3 He/ 4 He 50 R A FOZO is the major reservoir as everything mixes with FOZO (also see hauri et al., 1994)

34 Isotope arrays and mixing in the mantle large overlap between MORB and OIB compositions but mixing systematics very different Model well-mixed Model well mixed UM

35 Moving forward. A better handle on the initial conditions (maybe a statistical approach). Quantitative modeling on how the isotopic i arrays are produced. The amount of mantle entrained by the slab as it subducts into the lower mantle important not just for constraining mass flux but also for the thermal evolution.